There is already a known process for the preparation of isobutyryl fluoride from an anhydrous mixture of propylene, carbon monoxide, and hydrogen fluoride. In particular, U.S. Pat. No. 4,499,029 discloses passing such a mixture through at least two reaction zones arranged in series and adding to the reaction mixture, between the reaction zones, progressive quantities of anhydrous propylene and carbon monoxide, the process being carried out with a residence time of 15 seconds to 10 minutes in the reaction zones, at a pressure of 1 to 150 bars and a temperature of 0.degree. to 100.degree. C., the molar relationship C.sub.3 H.sub.6 /CO/HF in the reaction mixture being between 1/5/5 and 1/30/200. However, the cost of manufacture of isobutyryl fluoride produced in this manner is fairly high, given that the starting material in this process, namely propylene, is itself produced by dehydrogenation of alkanes from oil cuts or by steam cracking of hydrocarbons.
It is already known, from H. Hogeveen and C. F. Roobeek, Rec. Trav. Chim. Pays-Bas. 91 (1972), pages 137-40, to react at 0.degree. C. an equimolar mixture of n-butane and carbon monoxide in the presence of antimony pentafluoride SbF.sub.5 in solution in SO.sub.2 ClF. This reaction leads to the formation of a mixture of secbutyloxocarbonium (74%), tertbutylcarbonium (25%) and tertbutyloxocarbonium (1%) ions. From the same document, it is known to carbonylate propane at 0.degree. C., in a solvent (SO.sub.2 ClF) and in the presence of antimony pentafluoride, the molar relationship C.sub.3 H.sub.8 /CO being between 1 and 9. It is also known, from N. Yoneda et al, Chemical Letters (Chemical Society of Japan), (1983), pages 17-18, to carbonylate branched alkanes containing at least 5 carbon atoms, at a temperature of 30.degree. C., in the presence of the superacid HF-SbF.sub.5 (molar relationship HF/SbF.sub.5 equal to 5), the molar relationship alkane/HF being equal to 0.1. Furthermore, it is known from G. Olah et al, Journal of the American Chemical Society, 95, pages 4939 et seq., that:
At a temperature between -10.degree. C. and -103.degree. C., in a solvent (SO.sub.2 ClF) and in the presence of the superacid HSO.sub.3 F-SbF.sub.5, an equilibrium is established between propane and the isopropyl cation, and PA1 At a temperature of -78.degree. C., in a solvent (SO.sub.2 ClF) and in the presence of a superacidic system comprising hydrogen fluoride and antimony pentafluoride, an equilibrium is established between 2-methylpropane (or isobutane) and the trimethylcarbenium ion.
E. Hogeveen has already described in Adv. Phys. Org. Chem., 10, 32 (1973) the decarbonylation reaction of the pivaloyl cation at -70.degree. C., either in an equimolar mixture of hydrogen fluoride and antimony pentafluoride or in a mixture of 2 parts by volume of SO.sub.2 ClF per 1 part by volume of antimony pentafluoride, to form the tert-butyl cation.
It will be noted that, in general, these prior documents are concerned exclusively with the kinetics of protonation of alkanes or of decarbonylation at a very low temperature and do not describe any covalent species capable of being obtained by employing these reactions. In particular, none of them describe the production of acid fluorides. Furthermore, none of them have demonstrated the possibility of regenerating the superacid employed for the protonation.
Furthermore, U.S. Pat. No. 4,582,571 discloses the possibility of forming isobutyryl fluoride by reaction of carbon monoxide, propane, anhydrous hydrogen fluoride, and antimony pentafluoride at a pressure above 100 bars and at a temperature close to 100.degree. C. However, on the one hand this document is silent on the respective proportions of the various reactants and on the other hand discloses pressure and temperature conditions that are too severe for industrial use. Additionally, it has been discovered that the nature of the acid fluoride formed by such a process surprisingly depends on the value of the ratio CO/propane.